Conductive plastics for electrical and electronic applications.pdf

8/17/2019 Conductive plastics for electrical and electronic applications.pdf

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38 REINFORCEDplastics September 2005

Plastics are by nature very good

insulators. This inherent electrical

insulation causes the plastic to

tend to hold electrostatic charges and to

allow electromagnetic/radio frequency

interference (EMI/RFI) to pass through.

Recently, plastic has become the material

of choice for internals in electronic com-

ponents such as computers and other

consumer products, replacing metals, as

they offer greater design flexibility,

lighter weight, colorability and cost-

effectiveness. Thus, the challenge is to

convert inherently insulating thermo-

plastic materials to a product that would

provide antistatic or electrostatic

dissipative or EMI/RFI shielding or a

combination of these properties.

Managing heat is crucial to main-

taining the reliability and extending the

life of electronics. A wide range of

choice of cooling solutions is available

for shunting away excess component

heat, including fans, metallic or ceramic

heat sinks, pipes and spreaders. With

increasing demand for miniaturisation

of electronic devices such as laptops,

PDAs and other hand-held devices,

designers are now focusing on innova-

tive thermal management solutions

using design flexible, lightweight therm-

ally conducting plastics.

The Electronic Industries Associa-

tion (EIA) Standard 541 classifies con-

ductive plastics with respect to their

ability to protect against either electro-

static discharge (ESD) or electromagnet-

ic interference/ radio frequency inter-

ference (EMI/RFI). Materials with a

measured surface resistivity between

105-1012 ohms/sq. provides adequate

ESD performance, with lower conduc-

tivity products acting as antistatic prod-

ucts. Plastics with surface resistivity of


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39September 2005 REINFORCEDplastics

have also been used. Recently, more

emphasis has been on the use of carbon

nanofibres, where with small loading

high conductivities can be achieved sus-

taining plastic-like properties.

Although a wide variety of conduc-

tive fillers are available, the choice of filler depends on the end-user specifica-

tions in the target application. In some

antistatic applications, manufacturers

are looking for clear plastics, and thus

the additive or filler used should match

the refractive index of the polymer

matrix. In applications such as com-

puter chip and hard drive carrier trays,

the original equipment manufacturer

(OEM) is looking for conductive plastics

with low ionic concentrations. There-

fore, fillers used should not have leach-able ionics and other impurities at min-

imum levels. Lastly, both conductivity

and processability of these plastics

depend on the manufacturing process –

type of extruder used, screw configura-

tion, shear and temperatures employed.

The information presented here will

be focused on the use different surface-

active chemical agents, fillers such as

carbon fibres, carbon powders, carbon

nanotubes and combination of these

fillers to impart conductivity in plastics

especially in electrical and electronic

applications. Using mixed filler systems,

and utilising their interactions/syner-

gism, we were able to tailor the plastic

components to meet specific surface

resistivity range and processing require-

ments for different applications.

Antistatic plasticsStatic electricity is a common phen-

omenon that all of us have experienced –

clinging of hair when brushed and an

electrical shock obtained by touching a

doorknob or metal part. Static electricity

on a surface is generated due to the

build-up of charge. This could happen

either by a triboelectric effect – rubbing

sliding, or separating of nonconductiv

materials, or by an electrostatic field, cre

ated when one charged body induce

charge on a nearby second body. There

are many common processes wher

effective static charge dissipation is criti

cal to the device’s useful functions. On

example is the copying process. If the

components in the copier are not prop

erly grounded to dissipate charge, pape

jamming occurs. Plastics with antistati

properties are becoming increasingly

popular in applications such as printe

and copier components, vacuum cleane

dirt collecting cups, electrostati

painters, air cleaners, computer internal

and ink-jet printer penholders. Good

antistatic plastics should have highe

static decay rates. Antistatic plastics ar

widely used in media drives and in stor

age devices since these need rapid dissi

pation of surface charges generated dur

ing their operation (Figure 2).

Incorporation of antistatic agent

such as poly(amid-ether-ester) type poly

mers or organic alkyl sulfonates to th

plastics is one method of achieving th

required static dissipative properties

These types of polymers act as surface

active agents and impart a sligh

conductivity that is necessary for plastic

Conductive plastics for electrical and electronic applications

Figure 2. Antistatic plastics are widely used in applications where dissipation of static charge is needed, such as media drive cartridges. ( Picture courtesy of Plasmon.)

Figure 3. Effect of different types of fillers (at 10% loading) on PC-ABS resin. Metallic fillers or metal coated fibres provide higher conductivities with lower filler loadings.


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40 REINFORCEDplastics September 2005

to perform as antistatic agents. We have

found that the combination of these

types of chemical agents with carbon

fibres may provide a synergetic effect on

electrostatic decay characteristics.

Electrostatic dischargeElectrostatic discharge is a transfer of electrostatic charges between bodies at

different potentials caused by direct con-

tact induced by an electrostatic field.

Electrostatic dissipation has become an

important issue within the electronics

industry, especially in electronic compo-

nents such as data storage devices, chip

carriers and computer internals.

Different levels of surface and bulk

connectivity can be achieved with vari-

ous types of conductive fillers. Anincrease in conductive additive corre-

lates with a decrease in electrical resistiv-

ity and a critical threshold of fillers is

needed for complete and adequate dis-

charge. Volume resistivities obtained

using some commonly used fillers are

shown in Figure 3.

The changes seen in the resistivity

depend on the conductivity (type) of the

filler used, the degree of dispersion and

distribution, and the polymer system.

The most common fillers used in plastics

to impart electrical conductivity are

carbon powders and carbon fibres.

Although carbon powder products pro-

vide an inexpensive solution, for elec-

tronic applications these products are

not always attractive since they tend to

slough and deposit carbon on compo-

nents in contact with the plastic. As a

result, carbon fibres are widely used inconductive plastics, especially in electri-

cal and electronic applications. Carbon

fibres from different suppliers have diff-

erent characteristics due the sizing and

coupling agents used during their manu-

facture. As such, the plastic obtained

with same loading of carbon fibres from

different suppliers shows different con-

ductivity properties. Therefore, the selec-

tion of conductive filler type, processing

conditions and resin system used play an

important role in achieving the desiredsurface resistance values.

Conductive plastics are widely used

in electronic packaging applications,

such as chip trays and conductive carrier

tapes where IC chips are transported

from manufacturers to assembly plants.

These products require good control of

electrostatic dissipative properties cou-

pled with tighter dimensional tolerances

in pockets where chips are placed.

(Figure 4).

Recently nano-carbon fibres have

become popular since they allow higher

conductivities to be obtained even with

smaller loadings. For example 3% multi-

walled nanotube (MWNT) filled poly-

carbonate products exhibit the same

conductivity as 15% carbon fibre filledproducts (Figure 5), and with single

walled nanotubes (SWNT), the required

loading is less than 1%.

More and more conductive plastics

with MWNTs are now used in electri-

cal/electronic applications since they are

good electrical conductors with lower

filler loading. Furthermore, they do not

slough as carbon powder containing

products do. Although SWNTs give high-

er conductivities than MWNTs, this tech-

nology is still in early stage due to the

challenges in processing of these fibres.

The SWNT has strong van-der-Waal

forces and tends to form ropes. De-rop-

ing and good dispersion of individual

tubes in the plastic matrix are critical for

effective conductivities with SWNTs.

EMI shieldingElectromagnetic interference (EMI)

shielding is another property of import-

ance in many applications. Electronic

devices operating normally in their

intended environment, without con-

ducting or radiating excessive amounts

of electromagnetic energy, or not being

susceptible to such energy from internal

or external sources, are in the state of

electromagnetic compatibility (EMC).

EMI is radiated or conducted energy that

adversely affects a circuit’s performance,

and thus disrupts a device’s EMC. Many

types of electronic circuits radiate or are


Figure 4. Conductive chip trays use conductiveplastics with conductivity as specified by JEDEC standards. Chip trays need to berigid with no warpage after baking at set temperatures, usually at 150°C.

Figure 5. Effect of different types of conductive fibres in polycarbonate. Carbon nanotubes such as multi-walled nanotubes (MWNT) and single-walled nanotubes provided higher conductivities thancarbon fibres.


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4September 2005 REINFORCEDplastics

susceptible to EMI and must be shieldedto ensure proper performance.

EMI shielding using plastics can be

accomplished by using high-aspect ratio

conductive fillers such as carbon and

stainless steel fibres. Shielding is provided

by a conductive medium that reflects,

absorbs or transmits the electromagnetic

radiation to the ground. Shielding effec-

tiveness is determined by the extent to

which the intensity of an electromagnet-

ic signal is reduced by the introduction of

a shielding medium. To obtain good

shielding effectiveness, a higher filler

loading and good dispersion in the

plastics are required.

For EMI shielding plastics, metal-

coated fillers such as nickel-coated

carbon fibre or stainless steel fibres are

widely used. In order to effectively shield

electromagnetic waves the fillers used

should be able to reflect the radiation

from the plastic matrix. For these appli-

cations long carbon fibres that can form

an effective network in the polymermatrix are a better choice than short

fibres, as seen in Figure 6.

Thermally conductive plasticsWith electronic devices gaining power

with faster chips, speedy media drives

and hard disks, design engineers have

really started to feel the heat generated

in these systems. Thermal management

was once accomplished with a few well-

placed fans, vents and aluminium heat

sinks. But today’s small, hot electronicscan benefit from new ways to keep cool

like using thermally conductive plastics.

Whereas unfilled thermoplastics have a

thermal conductivity of around 0.2 W/

mK (Watts/meter-°Kelvin), most therm-

ally conductive plastic compounds typi-

cally have 10-50 times higher conduct-

ivity (1-10 W/mK). Similar to electrically

conductive plastics, thermal conduct-

ivity in plastics can be accomplished by

adding different thermally conductive

fillers. The most common fillers are

alumina-type ceramic fillers since they

are less expensive and impart isotropic

thermal conductivity (Figure 7).

Much higher thermal conductivity

can be achieved using speciality graphite

fibres made from petroleum pitch. Unlike

polyacrylonitrile (PAN)-based carbon

fibres, pitch-based carbon fibres have low

electrical conductivity but high thermal

conductivity due to their wavy radial

structure, and have conductivity values

of 500-1000 W/mK. By comparison,

structural-grade carbon fibres based on

PAN have conductivities less than 10 W/

mK. Other commonly used fillers include

boron nitride and aluminium nitride,

which are electrically insulative ceramic

fillers with a thermal conductivity of 60-

80 W/mK for boron nitride and 300 W/

mK for aluminium nitride powders.

Thermally conductive plastics are get-

ting increasingly popular in hard disk

drive internal components (Figure 8), lap

top computers, and other electrica

devices where placing cooling fans i

becoming impractical. These types oplastics are also used in larger device

such as computer base stations as hea

sinks to protect electronic components.

ChallengesPlastic composites play an importan

role in the electrical and electroni

industry for their optimum performance

as described here by controlling both

electrical and thermal conductivity. A

this industry grows, designers face mor

challenging requirements. These can

only be met through innovation of new

polymers and fillers systems and ways o

effectively combining these to obtain

maximum benefit.


Figure 6. The shielding effectiveness of PCwith different amounts of carbon fibres.Plastic pellets made from short carbon fibrehave random orientation of fibres, whereas long fibre pellets have fibres oriented on thefull pellet length. When parts are moulded,long carbon fibre products thus give enhanced shielding effectiveness.

Figure 7. Alumina (Al 2 O 3 ) filled PPS composites.

Higher filler loading is needed for effectivethermal conductivity with these types of ceramic fillers.

Figure 8. The spindle motor of a hard disk drive uses ceramic filler filled plastics to dissipate heat.

Dr Jay Amarasekera, GE Advanced Mat-

erials/LNP, 475 Creamery Way, Exton,

PA 19341, USA; e-mail: jay.amarasek-

[email protected].

This article was presented at the RP Asia

2005 conference in Bangkok, Thailand,

on 25-26 August ( www.

rpasia.com ). The pro-

ceedings from this con-

ference are available to

purchase; e-mail rp@

elsevier.com for details.
mailto:[email protected]:[email protected]://www.rpasia.com/http://www.rpasia.com/http://www.rpasia.com/mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]://www.rpasia.com/http://www.rpasia.com/

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